Flat-panel displays are widely used in conjunction with computing devices, in portable devices, and for entertainment devices such as televisions. Such displays typically employ a plurality of pixels distributed over a display substrate to display images, graphics, or text. In a color display, each pixel includes light emitters that emit light of different colors, such as red, green, and blue. For example, liquid crystal displays (LCDs) employ liquid crystals to block or transmit light from a backlight behind the liquid crystals and organic light-emitting diode (OLED) displays rely on passing current through a layer of organic material that glows in response to the current. Displays using inorganic light emitting diodes (LEDs) are also in widespread use for outdoor signage and have been demonstrated in a 55-inch television.
The various light-emitting technologies have different characteristics, advantages, and disadvantages. For example, liquid crystals are simple to control and have a highly developed and sophisticated technological infrastructure. Organic LEDs are area emitters, can be more efficient and flexible, and are demonstrated in a very thin form factor. Inorganic light-emitting diodes are very efficient and provide relatively saturated light in an environmentally robust structure. Lasers are also efficient, provide a virtually monochromatic light, but have a limited viewing angle. None of these technologies, however, meet all of a display viewer's needs under all circumstances.
Organic light-emitting diodes are widely used in portable electronic devices with displays and in some televisions. Organic LEDs are area emitters, can be efficient and flexible, can have a very thin form factor, and have an excellent viewing angle. However, the process used to manufacture OLED displays has some challenging steps. An OLED emitter typically includes several layers, for example a hole-injection layer, a light-emitting layer, and an electron-injection layer. The hole-injection layer is coated on a first electrode such as an anode and a second electrode such as a cathode is formed on an electron-injection layer. Alternatively, an electron-injection layer is formed on a cathode and the anode is formed on a hole-injection layer.
One type of OLED display is made with a common unpatterned light emitter for all pixels and patterned color filters that filter the light from each light-emitter in the display. Different color filters produce different colors and the common light emitter emits white light, for example a combination of blue and yellow light. This display type is similar to the color-filter approach found in LCDs and suffers from the loss of approximately two thirds of the emitted light in the color filters.
Another type of OLED display uses different organic material patterned over a display substrate. The different OLED materials are chosen to emit different colors of light and are patterned to form pixels, typically arranged in stripes. The strip pattern is formed by depositing organic material through a fine metal shadow mask. A different mask is used for each different set of materials, or at least for the different light-emitting layers. The alignment of the masks before deposition is difficult, and the repeated use of the masks can damage deposited materials. Moreover, the masks must be periodically cleaned, are easily damaged, difficult to make, and expensive.
There is a need, therefore, for devices, systems and methods for providing OLED light emitters that have improved efficiency, reduced costs, and fewer mechanical process steps.